You should have already specified which of your accounts you would like to use by default.
You can either choose which address should be used as soon as you open the Compose window — just make a selection from the From drop-down menu.
You can avoid this by unticking the box labeled Treat as an alias.
You’ll then be guided through the process of adding a new email account.
Enter the email address you’d like to pull emails from and click Next Step.
Click the Accounts link at the top of the Settings section and then scroll down to the “Check email from other accounts (using POP3):” heading.
Click the link labeled “Add a POP3 mail account you own”.
This will work in the vast majority of cases, but should you run into problems you can always return to this stage and opt for the manual option.